Here’s a design circuit for frequency modulator that
is completed with tuning circuit. In this circuit is using pair of 1N4007s are used as varactor diodes. I picked 1N4007s
not because they are particularly well suited, but simply that I had some on
the bench. One of the varactors is directly coupled to the xtal circuit through
1 nF so most of its capacitance change is seen, the other is coupled through a
small trimmer to adjust its maximum effect. This gives two independent
channels, one for relatively coarse frequency tuning, the other for FSK
modulation. Here’s the figure of the circuit;
The data looks
characteristically parabolic as expected. I suspect the oddness of the fine is
measurement error. The y coordinates are in Hz relative to 10.138000 MHz (x is
reverse voltage) so it doesn't take much error in measurement to make the fine
trace look lumpy.
